1. Field of the Invention
This invention relates generally to the production of maize, and more specifically to hybrid corn plants with certain advantageous phenotypes resulting from interactions of the haploid genetic contributions of inbred parental lines. Seeds and tissues, in particular, those capable of producing or regenerating the hybrid plants either in vivo or in vitro are disclosed. An aspect of this invention, hybrid DK743, is characterized by many advantageous phenotypic traits including excellent grain yield and silage production, staygreen and disease tolerance. Although these characteristics are particularly suited for southern climates, an advantage of this genetic complement is good performance of plants expressed over a wide geographical area. DK743 has characteristic restriction fragment length polymorphism (RFLP) and isozyme profiles.
2. Description of the Related Art
Crop improvement has been a major focus of human agriculturists since the hunting gathering societies moved into the agricultural phase of human existence. Early crude attempts to improve crops focused on the choice of parental plants to become the progenitors of the next generation, a choice made on the readily detectable characteristics of the parents. The objective was to produce offspring having the advantageous traits of the parents. However, from what we now know of genetics and genetic theory, such efforts were usually doomed to failure--in some instances because the parental phenotypes could not be reconstructed in their offspring due to disruption of the genetic complements of the parents by segregation of a diploid complement into haploid gametes, and shuffling of the genetic material by recombination. Even worse, certain combinations of parental genomes yielded deleterious effects due to interactions of genes at the same or different loci. As a consequence, success at crop improvement was painstakingly slow, sporadic and rarely reproducible.
Modern sophisticated crop breeding of the 1900's has benefitted from knowledge gained by Gregor Mendel, which was published in 1865 and rediscovered in the early 1900's. That work and subsequent work of others indicated that both single gene (mendelian) and polygenic control must be considered when planning breeding programs to improve crop characteristics. In fact, all corn as we know it today, Zea mays, is a result of human manipulation. It is not a plant occurring sporadically in nature.
Despite much knowledge that has developed subsequently, each breeding program represents at least in part a new attempt to mold the plant germplasm into new and more productive, more desirable phenotypes. This molding process benefits from the development over many years of inbred lines. These lines are not found in the wild, that is, in natural settings, and by themselves are generally not commercially productive. However, inbred lines are valuable as repositories for genes that are preserved in relatively stable conditions due to the true-breeding capabilities of these genetically uniform lines. Such genes are then available to be repeatedly tested for their effects in various breeding combinations and to be incorporated into commercially desirable crops.
Inbred lines are those that are essentially homozygous. Initial repeated inbreeding concentrates a subset of ancestral genes in offspring. Lines may also be improved in this fashion. The more genetically similar the ancestral lines, the more rapid the approach to homozygosity. Self-fertilization is a way of maintaining homozygosity. Homozygosity refers to the condition of the genetic complement in which the paired diploid positions at each locus are occupied by identical alleles. Alleles are conditions of a gene which differ in their nucleotide sequences. Alleles are recognizable when they are expressed as distinct phenotypes. Homozygosity in an inbred line is achieved by repeated inbreeding. In general, by the sixth or seventh generation, the inbred line is considered genetically pure, or "true-breeding" although spontaneous changes in the genetic material (mutations) and other events may preclude absolute homozygosity. Environmental variations may produce some phenotypic variability even within genetically identically lines.
Unfortunately, reduction in yield performance and the appearance of other plant characteristics which are undesirable accompanies inbreeding. In addition, progressive selfing (self-fertilization) of inbred lines reduces plant vigor. Many of these deleterious effects are caused by homozygosity for deleterious recessive genes whose effects are unmasked by loss of desirable dominant alleles. Consequently, inbred corn lines per se are not grown to be used as commercial crops. However, they are extremely important as vehicles to preserve genes and to produce first generation (F.sub.1) hybrids by the process of hybridization by cross-breeding.
Hybrid plants are likely to be heterozygous at many loci, as opposed to being homozygous, like the inbred parental lines. Heterozygosity refers to the fact that at a locus, there are different conditions of a gene (different alleles). One desirable result of crossing two inbred lines is that hybrid vigor or heterosis may arise wherein the hybrid plants produced have markedly improved higher yields, better stalks, better roots, better uniformity and better insect and disease resistance, than either inbred parent. For corn used as animal feed, one of the goals is decreasing the amount of feed needed for animal weight gain.
Furthermore, as result of self-pollination of these F.sub.l hybrid plants, a process possible in plants such as corn which have both male and female sex organs on the same plant or of cross-pollination of F.sub.1 hybrid plants, a second generation (S.sub.1 or F.sub.2) hybrid may be produced. Non-parental genetic combinations occur in these offspring due to independent assortment at meiosis of genes on different chromosomes and by recombination of genes on homologous (matched and paired at meiosis) chromosomes. Because of this further shuffling of the genetic material from the F.sub.1 into the F.sub.2, some of the F.sub.2 hybrid plants produce less desirable plants than those of the F.sub.1 in terms of the traits discussed above, due to homozygosity for deleterious recessive alleles and other disruption of the F.sub.1 genetic complement. In addition, there is increased variability overall of trait performance in the F.sub.2 due to this extensive genetic shuffling, in particular, if many loci are involved in controlling a particular trait. It is not generally beneficial, therefore, for farmers to save the seed of F.sub.1 hybrids. Rather, a cycle of purchase by farmers each year of F.sub.1 hybrid seed for planting is the rule. Seed corn companies attempt to market new improved seed each season to attract these consumers.
North American farmers plant over 70 million acres of corn at the present time. There are extensive national and international programs in commercial corn breeding. Clearly this endeavor has a major impact on humanity in the form of food production. Basic methods of cross breeding inbred lines to produce hybrids are known in concept by those skilled in the art. However, actual manipulation of these basic methods to generate improved hybrids is a delicate, arduous and sophisticated process. Breeders armed with methods to physically control plant breeding, and with an array of inbred lines with various known phenotypic traits, cannot expect to merely go into the field with these inbred lines, breed them using well-established general methods, and walk out of their laboratories, greenhouses and fields with superior hybrids.
One of the first difficulties encountered is in breeding superior inbred parental lines, due to the difficulties discussed above which are inherent in inbreeding, for example, reduced vigor.
The skilled corn breeder also must make determinations regarding which combinations of these inbred lines should be selected to produce improved hybrids. The traits selected for commercial desirability are generally not expressions of genes operating in a vacuum. Rather, to produce a plant which as a whole has an array of desirable characteristics, there must be a balance in terms of improvements. Phenotypic traits may show positive or negative correlations within inbred lines and between those lines and their hybrid progeny. Consequently, improving one trait may lead to poor outcome of another. Furthermore, hybrid plants that are beneficial in one set of environmental conditions may do poorly in others.
With the desperate need for increased food production within diverse areas of the world, and for growing various crops in different locations of the world for maximum and control of local persons over their agricultural destiny, it is important to develop wide ranges of hybrids that are going to perform well in both specific and general ecological and commercial niches.
Evidence of the difficulties inherent in commercial crop breeding is provided by the continual and highly competitive research in both the laboratory and the field revolving around improvement of inbred and hybrid lines. Removal of some of the uncertainty in large scale and expensive field testing is resulting from the application of methods of molecular biology whereby segments of the genetic complement may be singled out for faster, more selective and more successful breeding, and genetic complements may be combined in vitro, that is, in laboratory tissue culture vessels rather than in corn fields.
Some of the phenotypic traits for which improvements have continually been sought by hybridization of corn, include the yield; stalks; roots; resistance to insecticides, pests and disease; and markedly more uniform characteristics. With mechanical harvesting of many crops, uniformity of plant characteristics such as germination and stand establishment, growth rate, maturity, and fruit size, is important. Other desirable phenotypic characteristics for field crops include tolerance to heat and drought, reduced time to crop maturity, and better agronomic quality. However, despite some successes in breeding programs in the 1900's, progress is painstakingly slow--each qualitative improvement representing a small quantitative step.
Currently, it appears as if there is polygenic control over most commercially desirable traits such as yield. This means that many genes, generally on many chromosomes, contribute to the phenotypic appearance of the plant. The variance of the trait in inbred lines is less than that expected in hybrids formed from inbreds because of intralocus and interlocus interactions. Consequently, selective breeding programs to improve crops are not completely predictable. The development of inbred lines generally requires at least about 5 to 7 generations of selfing. Inbred lines are then cross-bred in an attempt to develop improved F.sub.1 hybrids.